The present invention relates to printers and copiers and in particular to printers and copiers that utilize heated intermediate transfer members.
Printers and copiers are well known. Modern copiers utilize powder or liquid toners comprising toner particles to form visible images. Generally, a latent electrostatic image is formed on an image forming surface (such as a photoreceptor). The image is developed using a toner (such as the aforementioned powder or liquid toners), and the developed image is transferred to a final substrate (i.e., paper). Often, the transfer is indirect; an intermediate transfer member (ITM) receives the image from the image forming surface and transfers it to a final substrate, usually by heat and pressure.
The need of heat and pressure in combination, for fixing and fusing the image onto the substrate arises from the particular properties of the toner particles, the carrier liquid and the substrate. In some liquid toners in which the toner particles solvate and are swelled by the carrier liquid. Good image transfer occurs when the following conditions are met:
1. just prior to transfer, the image is above the solvation temperature (generally, about 65-95xc2x0 C.), to produce swelling and softening of the toner particles and preferably to bring about coalescing of the toner particles;
2. as it is pressed against the paper, the image must be warm enough to penetrate the paper fibers and to bind to them (or to bind to a plastic or coated plastic substrate); and
3. while pressed against the paper, the image must cool sufficiently so that its adhesion to the ITM is less that the cohesion of the toner particles amongst themselves. Under this condition, and assuming that adhesion to paper is greater than that to the ITM, the image is transferred in its entirety to the paper with no cracking of the image and with no appreciable residue on the ITM.
In other words, a good image transfer is obtained when a suitable temperature versus time profile of the image is maintained.
This process was first described in U.S. Pat. No. 5,555,185, the disclosure of which is incorporated herein by reference.
In some systems, the substrate is in web form. In others, it is in sheet form.
In general, the systems described in the aforementioned patent and in other patents utilizing the same system rely on heating the ITM so that prior to transfer, the image temperature is higher than the solvation temperature. Generally, the ITM comprises a structure which allows the image to cool sufficiently during transfer. However, to assure good transfer, the image temperature must be 25-30xc2x0 C. higher than the solvation temperature (depending on the ink concentration) so that the image does not cool below the solvation temperature too quickly (i.e., before it binds to the substrate). Generally, the ITM comprises a blanket. When the external blanket temperature is at about 90-110xc2x0 C., the back of the blanket and the external surface of the ITM drum are much hotter, often by as much as 60-70xc2x0 C.
These relatively high operating temperatures place severe requirements on the materials used for the ITM blanket and reduces their operating life. Reducing the operating temperatures will improve life and increase the range of materials that may be used.
In U.S. Pat. Nos. 5,410,392 and 5,592,269, the disclosures of which are incorporated by reference, the opposite approach is taken. In these patents the paper is heated to a temperature above the solvation temperature prior to transfer. During the transfer the toner is heated by the paper and is fixed to the paper by beat and pressure. The paper cools by contact with the ITM during the transfer process.
One aspect of some preferred embodiments of the present invention relates to providing an imaging apparatus with a heated ITM and a pre-transfer heated substrate. By pre-heating the substrate to a temperature below the solvation temperature, the operating temperatures of the ITM and blanket can be reduced, when compared to those in the prior art, while maintaining a desired temperature versus time profile of the image during the transfer process. Furthermore, the good transfer properties achievable with a heated ITM are not only retained, but in many cases, transfer is actually improved.
In some preferred embodiments of the invention, the substrate is in web form, and pre-transfer heating takes place just upstream of the point of image transfer.
In some preferred embodiments of the invention, the substrate is heated by direct contact with a hot roller, pressed against it, upstream of the point of image transfer.
Alternatively, the substrate is heated by a radiant heater, positioned slightly over or under it, upstream of the point of image transfer.
Alternatively, the substrate is heated by a microwave radiator, positioned slightly over or under it, upstream of the point of image transfer.
Alternatively, the substrate is heated by a hot air blower, positioned slightly over or under it, upstream of the point of image transfer.
Alternatively, the substrate is heated by other heater as known in the art.
Each of the aforementioned methods of pre-heating of the substrate has certain advantages and certain disadvantages in terms of heating efficiency, safety, control features, simplicity of the design, freedom from malfunctions and uniformity of heating.
In some preferred embodiments of the invention, the substrate is in sheet form, and pre-transfer heating takes place when the sheet is on the backing roller, ahead of the point of transfer. Preferably, the substrate is heated by a hot air blower. Alternatively, the substrate is heated by a radiant heater. Alternatively, the substrate is heated by a microwave radiator. Alternatively, the substrate is heated by some other heater as known in the art.
Preferably, the substrate is cooled by a blower or other means after transfer of the image to it.
It should be understood that the reduction of temperature of the blanket may have other advantages, in addition to the increase in ITM life. It can also result in improved transfer from the intermediate transfer member to the ITM and/or savings in heater energy. For those systems in which the various separations are collected on the ITM and are transferred together to the final substrate, the lower temperature results in lower evaporation of carrier liquid from the separations on the ITM. Since the separations spend different amounts of time on the ITM, the separations have more nearly the same proportions of toner and carrier liquid when they are transferred to the final substrate. This apparently results in improved fixing on the substrate.
There is thus provided, in accordance with a preferred embodiment of the invention, a method of transferring an image on a surface to a substrate comprising:
(a) heating the surface to a first temperature above a temperature at which the image adheres to the substrate;
(b) heating the substrate to a second temperature above ambient temperature and below the first temperature;
(c) pressing the substrate to the surface;
(d) cooling the image while it is in contact with both the surface and the substrate such that it cools during said contact to a third temperature, below a temperature at which its cohesion is greater than its adhesion to the surface; and
(e) then separating the substrate from the surface, said image being transferred to the substrate.
Preferably, the third temperature is between the first and second temperatures.
Preferably, the second temperature is below the temperature at which the image cohesion is greater than its adhesion to the surface.
In a preferred embodiment of the invention, the image is cooled in (d) by transfer of heat from the image to the substrate, preferably substantially only by transfer of heat from the image to the surface.
Preferably the substrate is heated during said cooling of the image such that its temperature is greater than the second temperature.
In a preferred embodiment of the invention, the substrate is heated during said cooling of the image substantially only by heat transfer from the surface and from the image. Preferably the method includes cooling the substrate and the image thereon, after (e) to a temperature at least as low as the second temperature.
In a preferred embodiment of the invention, the temperature variation of the image while the surface is pressed against the image is such that the image remains at a temperature that is high enough for a time long enough to assure adhesion of the image to the substrate during separation of the surface from the substrate.
Preferably, the adhesion of the image after said cooling thereof to the substrate is greater than is its adhesion to the surface.
Preferably, the image is formed on an image forming member and transferred to said surface prior to subsequent transfer therefrom to the substrate, such that the surface is the surface of an intermediate transfer member. Preferably, the image forming member is a photoreceptor.
In a preferred embodiment of the invention, the image is formed by an electrostatic process.
Preferably, the image is formed by an electrophotographic process in which a latent electrostatic image is developed by a toner to form said image.
Preferably, the image is a toner image, preferably a liquid toner image. Preferably, the liquid toner image on the surface comprises toner particles and carrier liquid. Preferably, the carrier liquid at elevated temperatures above a solvation temperature and wherein the first temperature is above the solvation temperature. Preferably, the second temperature is below the solvation temperature. Preferably, the third temperature is below the solvation temperature.
In a preferred embodiment of the invention, the substrate is formed of paper. Alternatively, the substrate is formed of a plastic.
There is further provided, in accordance with a preferred embodiment of the invention, imaging apparatus comprising:
a heated image bearing surface having a toner image thereon;
an impression surface which is urged toward the image bearing surface to form an image transfer region therebetween;
a substrate transport mechanism which transports a substrate through the image transfer region at which said image is transferred to said substrate;
a heater that heats the substrate upstream of the image transfer region, such that it is at room temperature as it enters the image transfer region between pre-transfer heated substrate onto which the developed image is transferred.
In a preferred embodiment of the invention a desired temperature versus time profile of the developed image is maintained by controlling both the temperature of the intermediate transfer member and of the substrate.
Preferably, the apparatus utilizes the method of the invention.